The interpretation section of a geotechnical report

A geotechnical interpretation section is where measured ground data becomes advice a designer can build from. Everything before it is record keeping. The interpretative part takes borehole logs, in situ test results and laboratory data and turns them into a ground model, a set of characteristic values derived under Eurocode 7, and a statement of what remains unknown. Reviewers judge the whole report on this section, because it is the only part that commits to a view. A Geotechnical Assessment prepared for a planning application is expected by most authorities to carry a named, suitably qualified author, and professional review is strongly advised before it goes in.

Factual record and interpretation stay apart

Keep the factual material and the interpretation in separate, clearly labelled parts, even in a single bound document. The factual record is the exploratory hole logs, the field test results, the laboratory certificates and the monitoring data, presented without comment. The interpretation is everything that follows from them.

That separation is not presentational fussiness. If a later designer disagrees with your ground model, they need to re-derive from the raw data without unpicking your reasoning. And if the site is investigated again in five years, the factual record remains usable while the interpretation may be superseded.

Eurocode 7 sets up the same division through the Ground Investigation Report and the Geotechnical Design Report. The first collects and presents what was found. The second assigns design values and states the geotechnical design assumptions. For planning purposes you are usually writing something closer to the first plus an interpretative chapter, with the full design report following at the detailed design stage.

Building the ground model

The ground model is a set of layers with boundaries, properties and a described origin. It should read as a physical story of the site.

Say which stratum is made ground and how you distinguished it, because the boundary between made ground and natural deposits governs foundation depth more often than anything else on a brownfield site. State where you have interpolated between holes and where you have extrapolated beyond them. A model built from four boreholes on a one-hectare site is a sketch, and describing it as anything firmer invites trouble later.

Cross-sections earn their place here. Two sections at right angles, with the exploratory holes projected onto them and the inferred boundaries drawn as lines rather than certainties, communicate more than several pages of description. Where a boundary is uncertain, draw it dashed and say so in the caption.

Deriving characteristic values

Eurocode 7 asks for a cautious estimate of the value affecting the occurrence of the limit state. That phrase does the work, and it is not the same as the mean of your test results.

Set out the derivation explicitly:

1. List the test results for each stratum, with outliers identified rather than silently dropped.
2. Say whether the limit state is governed by a local value or by an averaged value over a failure surface, because that choice decides how cautious the estimate needs to be.
3. State the selected characteristic value and the reasoning, including any correlation used to convert a field test result into a strength or stiffness parameter.
4. Name the correlation source rather than referring to published relationships in general terms.
5. Give the partial factors and design approach you have assumed, so the designer knows what has already been applied.

A reviewer who cannot follow that chain will ask for the report to be reissued. Showing the arithmetic is the difference between a document that gets used and one that gets filed.

Groundwater and seasonal variation

Single-visit water strikes recorded during drilling are among the least reliable numbers in any ground investigation. Drilling fluid, cavity collapse and the time between striking water and taking a reading all distort them.

What the interpretation needs is monitoring data from installed standpipes or piezometers over a period long enough to catch seasonal change, ideally through a winter. Where that data does not exist, say so plainly and state the design groundwater level you have assumed and why. Perched water in made ground, which behaves quite differently from the regional water table, deserves its own paragraph on any former industrial site.

For basement schemes this section is often the one that decides the application. A Basement Impact Assessment will lean on your groundwater interpretation to predict whether the excavation alters flow paths or affects neighbouring structures, and a thin groundwater section will be sent straight back.

Stating residual uncertainty

Every interpretative section should contain a short, honest passage on what is not known. It should be a specific list, rather than hedging spread thinly across every parameter.

Typical entries are the lateral extent of a soft layer between widely spaced holes, the depth to rockhead outside the investigated footprint, the absence of a winter groundwater record, and the possibility of obstructions from a demolished structure. Then say what would resolve each one, and at what stage. A condition requiring supplementary investigation before construction is a reasonable outcome, and it is far better than an assurance that later proves wrong.

This is the same discipline that makes any evidence section defensible. Writing the sequential test section inside a flood risk assessment works the same way: state the method, show the steps, and be explicit about the limits of what you tested.

Where contamination findings cross over

Ground investigation on brownfield land usually serves two purposes at once, and the interpretative sections have to talk to each other rather than sit in separate volumes.

Gas monitoring results feed a gas risk assessment and any protection measures, and those measures then constrain foundation and slab design. Aggressive ground conditions from sulphate or chloride testing govern concrete specification. Where remediation involves excavating and replacing material, the geotechnical properties of the replacement become a design input. Contamination work is expected to follow BS 10175 and to be carried out by suitably qualified and experienced people, and the Contaminated Land Assessment for the same site should reference your ground model rather than build a different one.

Say in your interpretation which findings you have taken from the contamination work and which you have derived independently. Two reports quoting different made ground thicknesses for the same site is a familiar and entirely avoidable objection.

What a reviewing engineer checks first

Experienced reviewers do not read these reports front to back. They go to a handful of places.

They check whether the number of exploratory holes and their depths match the scale and category of the proposal. They look for the characteristic value derivation and whether it is shown or asserted. They check the groundwater basis. They look for the uncertainty statement, and its absence tells them something. Then they check whether the recommendations are usable, which means parameters and depths rather than advice to consult a specialist.

If you are commissioning rather than writing, those five checks are worth doing yourself before the report reaches the council. Where a review is needed from someone with the right competence, the professional directory is the route to a reviewer who can sign a draft off.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and a geotechnical interpretation is firmly in that category.